1. Field of the Invention
The present invention relates to a focus detecting method for estimating a focus condition by using a light flux transmitted through an image forming optical system in an optical instrument such as camera and microscope.
The present invention also relates to a distance measuring method for estimating a distance from an image forming optical system to an object in a distance measuring device installed in a compact camera or automotive vehicles.
2. Related Art Statement
There have been proposed various focus detecting devices for detecting a focus in optical devices such as camera and microscope and an image forming optical system is driven into an in-focused position by using a result of the focus detection. Among these focus detecting devices, there is a focus detecting device, in which a focus is detected by using a light flux transmitted through at least a part of the image forming optical system. This type of focus detecting device has been widely utilized, because there is no parallax in regardless of a distance from the camera to an object, any manufacturing error of an objective lens and any error in a movement of the objective lens toward an in-focused position can be corrected, and a precision of the focus detection can be attained even if the objective lens is exchanged as long as a part of a light flux emanating from the objective lens is made incident upon the focus detecting device. In particular, such a focus detecting device has been predominantly used in a single-lens reflex camera.
A method of detecting a focus condition by using a light flux transmitted through at least a part of the objective lens can be classified into the following two methods:
(1) Phase Difference Method: Two light fluxes transmitted through different portions of the objective lens are focused by a pair of lenses to form two images and focus is detected by estimating a difference in a distribution of light intensity of these images. This method is based on the fact that a distance between the two images is changed in accordance with focus condition. PA1 (2) Contrast Method: A contrast of an image formed by a light flux transmitted through the objective lens is estimated and a position of the objective lens at which a maximum contrast can be attained is detected as an in-focus position. PA1 a first step for dividing a light flux emanating from an object into a plurality of light fluxes which are transmitted though different regions of a pupil of the image forming optical system, receiving said plurality of light fluxes by a photoelectric converting means having a plurality of light receiving element arrays and deriving plural sets of photoelectrically converted output signals; PA1 a second step for selecting at least two sets of photoelectrically converted output signals among said plural sets of photoelectrically converted output PA1 a third step for adding said selected at least two sets of photoelectrically converted output signals at arbitrary positions of respective signals to derive a fist sum signal; PA1 a fourth step for adding said selected at least two sets of photoelectrically converted output signals at positions which are different from those in the third step to derive a second sum signal; PA1 a fifth step for performing contrast operation for said first sum signal to derive a first contrast value and performing contrast operation for said second sum signal to derive a second contrast value; and PA1 a sixth step for estimating a focus condition in accordance with said first and second contrast values. PA1 a first step for dividing a light flux emanating from an object into a plurality of light fluxes which are transmitted though different regions of a pupil of the image forming optical system, receiving said plurality of light fluxes by a photoelectric converting means having a plurality of light receiving elements and deriving plural sets of photoelectrically converted output signals; PA1 a second step for selecting at least two sets of photoelectrically converted output signals among said plural sets of photoelectrically converted output signals; PA1 a third step for adding said selected at least two sets of photoelectrically converted output signals at arbitrary positions of respective signals to derive a fist sum signal; PA1 a fourth step for adding said selected at least two sets of photoelectrically converted output signals at positions which are different from those in the third step to derive a second sum signal; PA1 a fifth step for performing contrast operation for said first sum signal to derive a first contrast value and performing contrast operation for said second sum signal to derive a second contrast value; and PA1 a sixth step for estimating a focus condition in accordance with said first and second contrast values.
At first, the phase difference method will be explained with reference to FIG. 1. As illustrated in FIG. 1, a condenser lens 3 is arranged near a predetermined image plane (predetermined focal plane, film conjugate plate) 2 of an image formed by an objective lens 1, a pair of separator lenses 11 are arranged behind the condenser lens 3, said separator lenses being separated from each other by such a distance that a desired precision of focus detection can be attained, and an array of photoelectric converting elements 12 arranged at a focus position of light fluxes emanating from the separator lenses 11.
When the objective lens 1 is in an in-focused position, an image I of an object is formed on the predetermined focal plane 2 and first and second images I.sub.01 and I.sub.02 of this image I are formed by the separator lenses 11 on a secondary focal plane which is perpendicular to an optical axis of the objective lens 1 and on which the array of photoelectric converting elements 12 is arranged. However, when the objective lens 1 is at such a position that an image F of the same object is formed at a plane in front of the predetermined focal plane 2, first and second images F.sub.01 and F.sub.02 are formed on a plane which is perpendicular to the optical axis and closer to the condenser lens 3 such that the first and second images F.sub.01 and F.sub.02 come closer to each other. This condition is generally called a forwardly focused condition. When an image B of the object is formed on a plane behind the condenser lens 3, first and second images B.sub.01 and B.sub.02 are formed on a plane behind the predetermined focal plane 12 such that these images are separated from each other in a direction perpendicular to the optical axis. This condition is called a backwardly focused condition. All the first images I.sub.01, F.sub.01 and B.sub.01 are directed upwards and all the second images I.sub.02, F.sub.02 and B.sub.2 are directed downwards. In the phase difference method, distribution patterns of light intensity of the first and second images on the array of photoelectric converting elements 12 are compared with each other and a focus position is detected by a result of this comparison. Focus detecting optical systems in accordance with the above mentioned phase difference method have been proposed in, for instance Japanese Patent Application Laid-open Publications Kokai Sho Nos. 55-118019, 58-106511 and 60-32012.
Next, the contrast method will be explained with reference to FIGS. 2 and 3. In FIGS. 2 and 3, a condenser lens 3 is arranged in a vicinity of a predetermined focal plane 2 of an objective lens 1 and an image reforming lens 13 is arranged behind the condenser lens 3 to reconstruct an image on an array of photoelectric converting elements 14 which is arranged at a conjugate position with the predetermined focal plane 2. FIG. 2 depicts an in-focused condition, in which a light flux transmitted through the objective lens 1 is focused on the predetermined focal plane 2. FIG. 3 shows a forwardly focused condition in which an focused image of the object is formed at a position before the predetermined position 2. In the in-focused condition, a sharp image of the object is formed on the array of photoelectric converting elements 14, so that the image has a high contrast. In the forwardly focused condition illustrated in FIG. 3, the defocused image is formed on the array of the photoelectric converting elements 14 and thus a contrast is low. Therefore, by moving the objective lens 1 in such a direction that a contrast of the image formed on the array of photoelectric converting elements 14 is increased, the objective lens can be driven into the in-focused position. In Japanese Patent Application Laid-open Publication Kokai Sho 63-127217, there is described that images are formed by a light flux transmitted through the objective lens 1 at positions before and after the predetermined focal plane 2, contrasts of these images are detected, and a focus condition is detected by comparing the thus detected contrasts of the two images. Further, the focal condition may be detected by moving the image reforming lens 13 along the optical axis.
There has been also proposed a camera system, in which a light flux emanating from an object is taken by using a focus detection optical system provided separately from an objective lens and a distance from the camera to the object is detected by processing the thus introduced light flux in accordance with the phase difference method. In this focus detection system, it is unnecessary to provide an optical member for dividing the light flux into image forming light and focus detection light or changing a light path, so that a whole camera system can be made compact and simple and it is particularly suitable for a compact camera system in which an objective lens is not exchanged.
In the focus detecting method based on the phase difference, a precision of focus detection might be decreased materially when an object to be picked-up has a periodic distribution in light intensity. That is, if the object has a fine stripe pattern, the focus condition could not be precisely detected.
In Japanese Patent Application Publication 5-32733, there is proposed a method of reducing a focus detection error or distance detection error for an object having a periodic distribution of light intensity. In this method, a focus detecting optical system is composed of three apertures which are arranged side by side and have different distances between centers of gravity or have different shapes, three image reforming lenses corresponding to said three apertures, and three arrays of photoelectric converting elements corresponding to said three image reforming lenses. Then, output signals from the three arrays of photoelectric converting elements are subjected to a correlation treatment, and a focus condition is detected. In this method, three sets of correlation operation can be performed from the three output signals produced by the three arrays of photoelectric converting elements. When all the three sets of correlation operation indicate an in-focus condition, it is determined that an in-focused condition is attained, but even if only one correlation operation shows a de-focused condition, it is judged that a focus condition could not be detected precisely although one or two remaining correlation operations indicated the in-focused condition. In this manner, it is possible to avoid an error in the focus detection or distance measurement due to a periodic distribution of light intensity.
However, in this known method, it is necessary to derive the three degrees of correlation and to find points having a higher degree of correlation. This point could not be detected by seeking a peak, because the the correlation degree is gradually changed in accordance with a change in a distance between the two images, so that it is necessary to calculate the correlation over a whole range. Therefore, this method requires a quite long time.
Moreover, in the known phase different method, the precision of focus detection is increased when an angle of a center light ray of the light flux to be used for the focus detection is increased. However, when this angle is large, the focus detection could not be performed for an objective lens having a large F-number.
Furthermore, in the known phase difference method, for a limited length of the array of photoelectric converting elements, precision and range of distance measurement are contradictory to each other. That is, when a distance measurement precision is increased, a measurable range is narrowed. Further, a range of distance measurement and a dimension of a field of view are also contradictory to each other.
Moreover, the known phase difference method has a demerit that the focus detection could not be carried out by taking account of influence of aberrations. It has been proposed to previously store information about influence of aberrations for an objective lens and the focus detection procedure is conducted on the basis of the read out information. However, it is practically impossible to store the information of influence of aberrations including manufacturing error and errors in stop positions of lens elements of a zoom lens. Further, it is very difficult to store information about influence of aberrations due to off-axis image.
Moreover, in the known phase difference method, although it is possible to detect a direction of defocusing, an amount of defocus could not be detected.